of clear wood parallel to grain. Table 5–7 lists average ten-
sile strength values for a limited number of specimens of a
few species. In the absence of sufficient tension test data,
modulus of rupture values are sometimes substituted for
tensile strength of small, clear, straight-grained pieces of
wood. The modulus of rupture is considered to be a low or
conservative estimate of tensile strength for clear specimens
(this is not true for lumber).
Less Common Properties
Strength properties less commonly measured in clear wood
include torsion, toughness, rolling shear, and fracture
toughness. Other properties involving time under load
include creep, creep rupture or duration of load, and fatigue
strength.
Torsion strength—Resistance to twisting about a longitu-
dinal axis. For solid wood members, torsional shear strength
may be taken as shear strength parallel to grain. Two-thirds
of the value for torsional shear strength may be used as an
estimate of the torsional shear stress at the proportional
limit.
Toughness—Energy required to cause rapid complete fail-
ure in a centrally loaded bending specimen. Tables 5–8 and
5–9 give average toughness values for samples of a few
hardwood and softwood species. Average coefficients of
variation for toughness as determined from approximately
50 species are shown in Table 5–6.
Creep and duration of load—Time-dependent deformation
of wood under load. If the load is sufficiently high and the
duration of load is long, failure (creep–rupture) will eventu-
ally occur. The time required to reach rupture is commonly
called duration of load. Duration of load is an important fac-
tor in setting design values for wood. Creep and duration of
load are described in later sections of this chapter.
Fatigue—Resistance to failure under specific combina-
tions of cyclic loading conditions: frequency and number
of cycles, maximum stress, ratio of maximum to minimum
stress, and other less-important factors. The main factors
affecting fatigue in wood are discussed later in this chapter.
The discussion also includes interpretation of fatigue data
and information on fatigue as a function of the service
environment.
Rolling shear strength—Shear strength of wood where
shearing force is in a longitudinal plane and is acting per-
pendicular to the grain. Few test values of rolling shear in
solid wood have been reported. In limited tests, rolling shear
strength averaged 18% to 28% of parallel-to-grain shear
values. Rolling shear strength is about the same in the longi-
tudinal–radial and longitudinal–tangential planes.
Nanoindentation hardness—This type of hardness mea-
surement is conducted at the nanometer scale (the scale of
the cell wall). Nanoindentation uses an extremely small
indenter of a hard material and specified shape (usually a
pyramid) to press into the surface with sufficient force that
the wood deforms. The load and deformation history is used
to develop mechanical property information. Nanoinden-
tion hardness provides a method for describing a material’s
response to various applied loading conditions at a scale that
may explain differences in wood cell structures and help
predict material performance after chemical treatments have
been applied (Moon and others 2006).
Fracture toughness—Ability of wood to withstand flaws
that initiate failure. Measurement of fracture toughness
helps identify the length of critical flaws that initiate failure
in materials.
To date, there is no standard test method for determining
fracture toughness in wood. Three types of stress fields, andChapter 5 Mechanical Properties of Wood
Table 5–4a. Mechanical properties of some commercially important woods grown in Canada and imported
into the United States (metric)a—con.Common species
namesMoisture
contentSpecific
gravityStatic bending Compression
parallel
to grain
(kPa)Compression
perpendicular
to grain
(kPa)Shear
parallel
to grain
(kPa)Modulus of
rupture
(kPa)Modulus of
elasticity
(MPa)
Engelmann Green 0.38 39,000 8,600 19,400 1,900 4,800
12% 70,000 10,700 42,400 3,700 7,600
Red Green 0.38 41,000 9,100 19,400 1,900 5,600
12% 71,000 11,000 38,500 3,800 9,200
Sitka Green 0.35 37,000 9,400 17,600 2,000 4,300
12% 70,000 11,200 37,800 4,100 6,800
White Green 0.35 35,000 7,900 17,000 1,600 4,600
12% 63,000 10,000 37,000 3,400 6,800
Tamarack Green 0.48 47,000 8,600 21,600 2,800 6,300
12% 76,000 9,400 44,900 6,200 9,000
aResults of tests on clear, straight-grained specimens. Property values based on ASTM Standard D 2555–88. Information on additional
properties can be obtained from Department of Forestry, Canada, Publication No. 1104. For each species, values in the first line are from tests
of green material; those in the second line are adjusted from the green condition to 12% moisture content using dry to green clear wood
property ratios as reported in ASTM D 2555–88. Specific gravity is based on weight when ovendry and volume when green.